Rain water, dehumidifier condensate and R/O are all in the same boat, completely missing calmag. I use both R/O and rain water and I am grateful that we dont have city water here. In addition to fluoride, chlorine and chloramines, tap water may deliver "emerging contaminants" of primary concern are the hundreds of pharmaceuticals found in our rivers, which are not filtered out by granular activated carbon filters or sediment filters. Probably not a huge issue but R/O will filter them out if they are present and if you have a modest size grow no problem to use R/O. Rain is a great way to go, mine comes off the roof at about 5 ppm TDS.
Anyway, blackstrap molasses provides some calmag and iron but also a strong dose of K to keep in mind. You can soak R/O or rain water in pulverized dolomite lime to pick up a bit of calmag. Large amounts of plant based compost never hurts. You could also use a combination of epsom salts, gypsum and oyster shell flour in your water to get highly soluble cal and mag. 100ppm of those in your water is probably a good place to start if your soil already has gypsum, oyster shell and good compost in it.
Another option is a green leaf slurry. Soak the ground up/mashed up leaves in rain water or R/O and they will pick up a steady release of many nutrients from the leaves. Plants and soil microbes really seem to love that in my experience.